The present invention relates to a cochannel interference detection system in radio communicatins. The present invention is applicable to a mobile communication, and/or cordless telephone and other radio communication systems. The present invention is applicable to the radio communication system using an angle modulation system (FM or PM).
One of the prior systems for detecting cochannel interference uses a pilot signal for detecting interference. In this system, a transmitter transmits power modulated by a pilot frequency which is set outside of the baseband, and the frequency of the pilot is different for each zone. When a receiver receives a pilot signal which is assigned to another zone, it is judged that there is an interference. However, that system has the disadvantages that the baseband spectrum is spread, the structure of the system is complicated (a transmitter must have a pilot signal generator, and a receiver must have a pilot signal receiver), and it can only detect an interference when D/U (ratio of desired signal to undesired signal) is smaller than several dB.
Additional prior interference detection equipment is shown in U.S. Pat. No. 4,085,370 which uses noise in a baseband generated by interference. This system can operate correctly only when the reception level does not widely vary and the modulation index is almost constant. However, it can not detect the average of the D/U ratio.
Further, prior interference detection systems have the disadvantage that they can merely detect the presence of interference, they can not measure the quantity of interference. Therefore, when communication quality becomes poor becouse of interference, the automatic switching has been almost impossible.